Refrigerated display cases which have a front opening portion are well known in the prior art. In the prior art, multiple air curtain construction is utilized in the display case to isolate the refrigerated space from the ambient atmosphere. In this way, the refrigerated foods or the like can be easily removed from and replaced in the refrigerated display case. The display case employs an innermost air curtain and a second adjacent air curtain which are normally circulated within the display case through conduits provided therein. The innermost air curtain is normally the coldest and the second curtain is somewhat warmer. A refrigerating means, typically in the form of one or more evaporators, is located within the innermost curtain passage for cooling the air flowing therethrough.
In this type of refrigerated display case, the innermost curtain passage and the refrigerating means must be frequently defrosted to remove the accumulated frost on the evaporators collected from the cooled air which impedes the operation of the equipment. On a commercial unit, such a defrosting operation is achieved by the use of high voltage electrical heaters located adjacent to the evaporator of the refrigerating means, or in some instances, by passing a hot gas through the evaporator of the refrigerating means. However, the hot gas defrost method is complex in its construction and is practical in only a small percentage ofinstallations. With the electrical heater defrost, the refrigerating mechanism is temporarily heated while allowing the circulating air through the air curtains. Thus, the circulating air is warmed by the electrical heaters. The warm air can then melt the frost built up on the evaporator. It is important to melt this frost as rapidly as possible in order to minimize the temperature rise of the refrigerated foods and to minimize collection of frost on the refrigerated foods from the higher humidity in the recirculated warm air.
To resolve the above mentioned problems, a dual evaporating system is disclosed in copending and commonly assigned application Ser. No. 765,216, filed on Aug. 13, 1985, U.S. Pat. No. 4,633,677. Said application is incorporated herein by reference. The refrigerating means disclosed in application Ser. No. 765,216 includes two evaporators both of which are placed on respective air ducts. These ducts are positioned in line with the innermost air curtain flow and the evaporators are parallel and serial connected with one another by control valves which are operated in accordance with the storage space temperature and defrosting requirements of the evaporator. One of the two evaporators functions as an evaporator to refrigerate the passed air while the other evaporator is defrosted by hot gas. Therefore, defrosting of the evaporators can be achieved while maintaining the temperature in the storage space.
In the initial stage of operation of the refrigerating means, the temperature in the storage space is not cold. Therefore, the two evaporators are normally operated as heat exchanges to quickly cool the passed air for rapidly cooling the storage space. Also, in comparison with normal operation, initial start up of the refrigerating means is under a large load. This is because the two evaporators are started up at the same time. Therefore, the compressor in the refrigerating means has a large starting load. Under these conditions, the refrigerating system is not efficient to operate and is more likely to fail prematurely.